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Thread: Help explaining 208v system with compressor & capacitors

  1. #1
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    Help explaining 208v system with compressor & capacitors

    Howdy folks!

    Was doing a maintenance the other week and was looking at a wiring diagram on a condensing unit and struggled to understand how the electricals worked.
    System is running fine and has no issues but can't get my head around this one!

    I'm having trouble understanding why my start winding has 208v going to it when the compressor is running. I would think the potential relay cuts out the start winding or voltage to it.
    After the compressor start up I only have 120v at my run winding & 208v at start winding :S

    Tried reaching out to my colleague but he also couldn't fully understand.

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  2. #2
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    Where are you checking this voltage to? Just saying that the start winding had X volts tells nothing, it need a point of reference like Start to L1 or Start to ground, etc. When checking volts, the meter is telling you the potential between the 2 leads. So depending on where the leads are placed makes a difference on what it is telling you.

  3. #3
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    What voltage is this cond unit operating on? 110vac?

  4. #4
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    It’s called back feed!

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  6. #5
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    Quote Originally Posted by TechmanTerry View Post
    What voltage is this cond unit operating on? 110vac?
    I’m guessing it’s 110v says separate circuit for cch
    Honeywell you can buy better but you cant pay more

    I told my wife when i die to sell my fishing stuff for what its worth not what i told her i paid for it

  7. #6
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    If we only had a compressor model number

  8. #7
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    The thread heading says 208v ....

    He must be checking ground to terminals

  9. #8
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    Quote Originally Posted by Snapperhead View Post
    The thread heading says 208v ....

    He must be checking ground to terminals
    Hes checking Common to run.

    Thats getting back feed thru the run terminal!

  10. #9
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    Quote Originally Posted by BNME8EZ View Post
    Where are you checking this voltage to? Just saying that the start winding had X volts tells nothing, it need a point of reference like Start to L1 or Start to ground, etc. When checking volts, the meter is telling you the potential between the 2 leads. So depending on where the leads are placed makes a difference on what it is telling you.
    Spot on. Most people look at you cross eyed when you tell them a volt meter doesn't actually tell you voltage. Try puting both leads on the same hot leg. It will tell you 0 volts - but don't touch it though because you'll get it! Meters indicate the difference in voltage between the leads. So as was stated, the OP needs to tell us test points to make any sense of readings. Also never troubleshoot to ground, it will often throw you for a loop. It is only useful for determining if a test point is safe to touch or not.

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  12. #10
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    The real problem here is the way they draw those schematics. I know exactly what is going on here, but trying to follow that schematic is a real mind bender. Once in awhile I'll come across a unit that was mis-wired by a lesser tech. I always start by drawing my own ladder schematic to follow. I remember the last one was several months back, just pulled all the wires off and started from scratch, ended up not needing a couple of the 'extra' wires.
    I do a triple evac with nitro to remove non condensables.

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  14. #11
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    Thread Starter
    208v

  15. #12
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    Quote Originally Posted by BNME8EZ View Post
    Where are you checking this voltage to? Just saying that the start winding had X volts tells nothing, it need a point of reference like Start to L1 or Start to ground, etc. When checking volts, the meter is telling you the potential between the 2 leads. So depending on where the leads are placed makes a difference on what it is telling you.
    Intially to ground but went back and tested further for y'all to see

    COMPRESSOR WHILST RUNNING

    Ground to #5 on Potential Relay = 120v
    Ground to #1 on Potential Relay = 120v
    Ground to # 2 on Potential Relay = 120v

    Start Capacitor
    Ground to either terminal = 120v
    L1 (Common) to either terminal = 208v

    Run Capacitor
    Ground to either terminal = 208v
    L1 (Common) to either terminal 250v yep..... 250v

    L1 (Common) to Start winding = 250v

    L1 (Common) to Run winding = 208v

    L2 to Run winding = 0V

  16. #13
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  17. #14
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    EDIT: For those looking for more information see below.

    Thank you!


    COMPRESSOR WHILST RUNNING

    Ground to #5 on Potential Relay = 120v
    Ground to #1 on Potential Relay = 120v
    Ground to # 2 on Potential Relay = 120v

    Start Capacitor
    Ground to either terminal = 120v
    L1 (Common) to either terminal = 208v

    Run Capacitor
    Ground to either terminal = 208v
    L1 (Common) to either terminal 250v yep..... 250v

    L1 (Common) to Start winding = 250v

    L1 (Common) to Run winding = 208v

    L2 to Run winding = 0V

  18. #15
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    What do you know about Back-EMF ?

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  20. #16
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    Quote Originally Posted by flangatang7 View Post
    While running

    Run Capacitor
    Ground to either terminal = 208v
    L1 (Common) to either terminal 250v yep..... 250v

    L1 (Common) to Start winding = 250v
    That 250 tells you how powerful of a capacitor you need .... like a 370 will work fine ( 5 - 45 - 370 ) for example , because it can handle up to 370v Back EMF volts

    Some units when taking that measurement you will see 420v , and in that case you need a capacitor thats 440 ( 5 - 45 - 440 )

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  22. #17
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    Quote Originally Posted by TechmanTerry View Post
    What do you know about Back-EMF ?
    Not a whole lot unfortunately.

  23. #18
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    How about looking at Potential Relay's and the Pick-up/Drop-out/Continous coil Voltages rating info for each diff relay. Some compressors produce Back-EMF Voltages(continous coil volt) in 300-400-500v range.
    How about looking at the Compressor (2) windings ,Main(run) & Start winding. 2 winding of two diff lengths & of two diff thickness produce two diff out of phase magnetic fields and out of phase voltages, which are then added together (just like a 208v system has two out of phases legs L1 &L2. Now KindaSorta each leg is 104v to ground, 104 + 104=208v) So 208v + 200Back-emf = 408v at the start winding. KindaSorta.

  24. #19
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    Quote Originally Posted by flangatang7 View Post
    Not a whole lot unfortunately.
    When you probed the Run capacitor from C - Herm .... it gave you 250 volts

    Thats your back EMF voltage

    Its the voltage thats produced under a load

    This information isnt really helping solve your problem ... but its something you need to know as a tech

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  26. #20
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    I have gotten bad motors straight out the box ....

    Maybe you finally got yours

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